refactor(drivers): delete GetMemoryFootprint / FlushOptionalCachesAsync (#525)

Both `IDriver` members had no production call site. Their only consumer was
`MemoryTracking`, deleted with the driver-tier recycle machinery in #522, and
`GetMemoryFootprint`'s doc-comment claimed "polled every 30s by Core" — which
had not been true since v3, even before #522 removed the poller.

Owner decision on #525 was (a) delete rather than (b) build the process-wide
watchdog that would consume them. The driver-side implementations are in git
history and were mostly reusable; what was always missing is the trigger, and a
member nobody calls is how the misleading doc-comment survived this long.

Removed from `IDriver`, all 12 driver implementations, and every test stub —
plus the support code that existed only to feed them, which is the part a
member-only deletion would have left behind as dead weight:

- MTConnect's three per-entry byte-weight constants, its cached
  `ProbeModelDataItemCount`, and `CountDataItems` — a full device-tree walk run
  on every cache fill to populate a number with no remaining reader.
- OpcUaClient's `_discoveredNodeCount` field and the discovery-pass write to it.
- MQTT's `ApproxBytesPerAuthoredTag`.

Two behavioural notes, neither a change in what the software does:

- `AbCipTemplateCache` now has no production reset path. Its only caller was
  AB CIP's flush, which itself had no caller — so nothing changes except how
  visible the fact is. Recorded on the type, along with the correction that its
  comment's claim about a device `ReinitializeAsync` also flushing it was never
  true. A UDT redefined on a live controller keeps its stale shape until the
  driver instance is rebuilt.
- MTConnect's `/probe` cache is still dropped, by `InvalidateProbeModel()` on an
  Agent restart. That is now `internal` rather than `private` so the tests which
  used the flush purely to empty the cache drive the real path instead — they
  now cover a case that actually occurs in production. Several unrelated doc
  comments cited `FlushOptionalCachesAsync` as the canonical CAS example and are
  repointed to it.

Tests: the ones that existed only to assert on the removed members are deleted
(3 OpcUaClient, 3 Galaxy, 2 MQTT, 1 MTConnect, 1 AbCip). Worth noting one was
vacuous — `FlushOptionalCachesAsync_resets_footprint_counter` asserted a
footprint of 0 before the flush and 0 after with no discovery in between, so it
would have passed against a flush that did nothing at all.

Full suite green except 7 failures verified identical on master before and
after (4 AbLegacy integration with the compose profile down, 3
Host.IntegrationTests), checked by name rather than assumed.

Docs corrected where they described this as live: `DriverLifecycle.md` (which
carried the #525 placeholder), `drivers/AbCip.md`, `drivers/AbLegacy.md`, and
`v2/driver-stability.md` — whose "polls this every 30 s" + per-driver
`DriverConfig` memory budget were never built as written and are now banner-ed
as design intent rather than description.

Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo

docs/v2/driver-stability.md

@@ -120,6 +120,8 @@ For Tier A/B (in-process) drivers, the "crash" being counted is a driver-instanc
 
 In-process drivers cannot be recycled by killing the server process — that would take down every other driver, every session, and the OPC UA endpoint. RSS watchdogs and scheduled recycle therefore do **not** apply to Tier A/B. Instead, each driver instance is monitored at a finer grain:
 
+> ⚠️ **The two bullets below were never built as described, and the surface they name is now gone.** `GetMemoryFootprint()` / `FlushOptionalCachesAsync()` did exist on `IDriver`, but nothing ever polled the footprint on a 30 s cadence in v3, no driver ever declared a cache budget in `DriverConfig`, and the recycle machinery their only consumer belonged to was deleted in Gitea #522. Both members were removed from `IDriver` in Gitea #525. Retained here as the design intent a future process-wide watchdog could revive — not as a description of the system.
+
 - **Per-instance allocation tracking**: drivers expose a `GetMemoryFootprint()` capability returning bytes attributable to their own caches (symbol cache, subscription items, queued operations). The Core polls this every 30 s and logs growth slope per driver instance.
 - **Soft-limit on cached state**: each driver declares a memory budget for its caches in `DriverConfig`. On breach, the Core asks the driver to flush optional caches (e.g. discard symbol cache, force re-discovery). No process action.
 - **Escalation rule**: if a driver instance's footprint cannot be bounded by cache flushing — or if growth is in opaque allocations the driver can't account for — that driver is a candidate for **promotion to Tier C**. Process recycle is the only safe leak remediation, and the only way to apply process recycle to a single driver is to give it its own process.